This was my entry for a recent contest involving 555 timers (now also an entry for the Open [18+] category for the Robot week Contest) and I wanted to share how to build it! It's a robot that can be set free on any ferrous metal structure. It has rare earth magnets for wheels (covered in hot glue for traction) attached to micro servos modified for continuous rotation. The bot closely resembles the Beetlebot by Jerome Demers (aka Robomaniac), as it uses limit switches to change the direction of the bot. When the switch closes it shorts the connection and cause the servo pulses to manipulate the direction of the servo. I used a 555 timer for each servo to set up the 1.0 – 2.0 mS signal needed. I used servo tester boards from Gadget Gangster that I modified to suite my needs. Here's a video of the bot in action:

Step 1: Materials Needed

Material List:

- Gadget Gangster Simple servo testers
- Neodymium Disc Magnets
- HiTec HS- 55 Micro Servos (Pretty common micro servo at most Hobby shops)
- Limit Switches
- 9v Aluminum Battery Holder
- Antenna wire are salvaged from a Wok Box handle (very solderabe)
- Female Header cables
- Automotive Terminal Connectors
- LM2937 5V Voltage Regulator (a 7805 will do)
- Caps for the regulator and 820 ohm & 15 kohm resistors
- 9V Battery
- Plastic Bead

Tools:

- Soldering Iron
- Flush Cutters
- Flux Pen
- Hot glue gun
- Needle Nose Pliers
- 1.35mm Drill Bit (or somewhere close to that size)
- No. 0 Philips Scredriver
- No.1 Flathead Scredriver
- Solder Wick
- Tweezers
- Heat gun or Hair Dryer
- Utility or Hobby Knife
- Multimeter

P.S. This instructable will assume you know how to solder, if you don't, please check out info on other instructables, MAKE blog, & Solarbotics as they inlude a tutorial in most of their kit documentation.

Step 2: Prep the Simple Servo Testers

If you were like me and wanted to test the Simple servo testers before you start messing around with them, you'll have to desolder the 2 potentiometers off the circuit board(s) and a header strip. In the end you'll want both boards to look simple to the first picture below, which has both Pots removed and an 820 ohm resistor and 3 pin header added in on the left side. Notice the top 2 pins on the header are shorted together, yes, this is on-purpose, see the added diagram (last photo on this step) for details. My diagram was modified from the original Simple Servo Tester Diagram.

Step 3: Prep Your Limit Switches

Now it's time for a switch...er...I mean change.....a change to switches.

Now in the following order do these steps:

- Use a heavy duty set of pliers to cut the excess metal off the switch's bumper rail
- If the excess metal still stays try wiggling it back and forth until the metal weakens and breaks off
- Pull the plastic part off the automotive terminal
- Sqeeze the terminal a bit if it's a bit larger than the bumper rail
- If you sqeezed too much open the terminal up a bit with the push of a flathead screwdriver
- Fit the terminal onto the switch but not too far, you don't want it touching the plastic
- Solder your solderable antenna wire to the terminal (not pictured, sorry)

Repeat these steps for the other switch

Now solder on your 15Kohm resistor and wire with 2 position female header on the end. One switch will have the resistor (and wiring) running from Normally open (NO) pin to the common pin while the other will have it going from the Normally closed (NC) pin to the common pin. The reason for this is that the servo are facing opposite of each other so we will need to send opposite signals to each in order for the bot to move forward instead of spinning in circles.

Step 4: Now Pin the Tail on the Botty....Body

Place Plastic Bead on some left over antenna wire, take your needle nose pliers and bend it at two 90 degree angles to trap the bead between them. This will be the tail dragging roller ball so your bot isn't dragging it's butt around.

Step 5: Modify Your Micro Servos for Continous Rotation

Here are the following crucial steps tp making this bot work, I derived this method from a very clever hack at Letsmakerobots.com

- With your No. 0 Philips Screwdriver take out all the screws in the Servo
- Use some tweezers to gently pry the top servo gear off the potentiometer then you can easily take the rest off careful not to break gears we need them all intact and try to remember the order that they came off
- Now use a 1.35mm drill bit or equivalent to drill out the flat spot inside the top gear, you can just spin the bit by hand
- Look inside the servo lid and you will find a limit point, trim this flush with a pair of flush cutters
- Go back to your main servo body and carefully wiggle the 3-pin potentiometer out of the housing (still wired in)
- With a multimeter measure the resistance of each side of the potentiometer and tweak until you get it close to a balanced position
- Now when the Pot is in the center position take your soldering iron and solder a dab of solder on the backside of the pot between the center pin and the metal that surrounds it, this will lock the potentiometer in the center position allowing the servo to be used for continuous rotation
- Put the Pot back in the housing and start putting your gears back on, make sure the top gear can spin freely on the now locked potentiometer shaft, if it doesn't drill it out a bit more with a slightly larger bit.
- House the servo up and then test it with your Simple servo tester board (4.8V to 6V applied)

Repeat this step for the second servo

Step 6: Make Your Wheels

You can make a little mangetic mount if you like to make this step easier.

Pour Hot glue all around the outer edge of each magnet. Try for as thin as you can, 1mm is pretty good. To get the layer thinner you can use a Heat gun to re-melt the hot glue and then use a piece of cardboard to scrap away the excess glue and smooth it at the same time. Don't get the Heat gun tip too close to the magnet cause it may fly off the mount and get stuck to the hot surface of your gun. With this step be careful not to handle the magnets right away after they've been heated cause they'll still be quite hot!

After wheels are made dab some hot glue on one side of each and push them against the servo horn, one at a time on each side. Try to get them as centered as you can by watching the space between the edge of the horn and the edge of the magnet in all four quadrents. If you don't get it right, try again by remelting with the heat gun, just watch that you don't mess up the outer edges that you worked so hard to perfect.

Step 7: We Come Together, Right Now, Over Free

It's time to put all the subsections together, yay!

A portion that is not pictured is the switch and regulator circuit, I placed these just under the Battery holder at the front of the bot, the wiring diagram of this circuit is pictured along with the wiring diagram of the entire bot after it (sorry it's not very easy to see, but you get the gist of it).

I didn't end up documenting the way I put it all together but I don't think it matters that much anyway, it was just a bunch of hot glue and besides I'd like to see some bots that are uniquely different than mine. Maybe someone will even solder the entire circuit on a protoboard and save the cash on the servo testers, that'd be sweet. I'm sure some cheap 555 based protoboards will start appearing soon after the hype of the 555contest so it might make things easier. The sky is the limit...well maybe the edge of your metal surface of choice in this case :)

Good luck with your robots, and if you build this please let me know what kind of things you've done with your bot, I'd really like to hear about it!